Bras, especially when worn daily, can have a major influence on your shape. Especially when you consider most women don't ever wear them for as little as the suggested 8 hours, and instead confine them for 12-14 hours a day where they can't breathe and get circulation (some women even wear bras to bed every night!)
I have a friend who has always worn a big push-up, and she prefers to wear it with the adjustable straps coming behind her neck instead of straight down her back. She's got some very noticeable armpit fat from it, even though she's a very petite girl (she's less than 5' tall and the rest of her body is fit to match). It hasn't affected her shape other than that, and I'm doubtful that push-ups will reshape your full breast tissue. Still, it will make slight changes in the area surrounding your breasts when you're wearing the wrong size.
That's a great site, and it's got so much information on finding your real bra size, as well as some very inspiring transformations!
I always wear soft, non-padded bras, because I like my breast shape and it's very comfortable to me--like a second skin. I actually tried bras on yesterday and was appalled at some of the bra shapes out there. Some of them that were my actual bra size were shaped so strangely that my breasts would never be able to fill it out properly. Also, my only bra that kind-of fits just broke yesterday, and I wore my backup bra that's a size too small, and it wasn't even close to fitting. I can't even wear it anymore. The cups don't come out nearly far enough to hold my entire breasts, and not only was it painful, but over time, something like that would definitely push my boob fat into my back.
This is why Tigerlily's method involves pulling fat from your back into your bra cup. Over time, the fat will transfer into your breasts and become breast tissue (although for some people, I guess this would be them regaining breast fat that they've lost).
Although I didn't realize people could fit such small bands so easily. I know 32 is more my band size than 34, and it's pretty tight against my wide ribcage, but I wonder if I might even fit a 30 band better for this sizing. I've read you're not supposed to be able to snap your bra on the tightest hook from the beginning, and that that hook is only meant to be used so your bra will still be snug as it stretches over time. Well, a 32 band fit on the tightest hook yesterday (tightly, but not too painfully). Maybe a 30 band will give me an even better fit and help shape my boobs around my armpits even more.
I'm still skeptical about the push-up thing, but maybe it does affect firmness of the breasts some. I don't ever wear push-ups; I've never even wore a real padded bra, so I can't say for sure what those really might do for your breasts.